1. Field of the Invention
The invention relates to digitally controlled variable power amplifiers for radio frequency signals, and more particularly to variable power amplifiers for providing accurately tapered power levels to individual antenna elements of a phased array radar system for sidelobe control.
2. Prior Art
Active array sidelobe control in transmission is an accepted means of enhancing survivability and anti-jam performance of a multi-mode agile beam radar. The conventional approach is to design the power amplifier for optimum power-added efficiency when saturated and accept the degradation in efficiency when tapering is implemented via linear operation. Unfortunately, most power amplifiers incur a significant insertion phase change from linear to saturated operation that is extremely sensitive to the gain compression level. The net result is that even the central radiating elements must be backed-off by 3 dB. This loss in addition to taper losses of 1.0 to 1.5 dB results in a significant degradation in performance.
Dual gate devices have also been explored as variable gain power amplifiers but the analog control of the second gate creates formidable linearization and calibration requirements in large arrays.